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Difficulty: EasyNeoplastic Processes

A 68-year-old woman is brought to the emergency department by her family due to progressive confusion, weakness, and severe constipation over the past week. She has a 40-pack-year smoking history. Her vital signs are within normal limits. Physical examination reveals lethargy and diminished deep tendon reflexes. Laboratory studies reveal:

Serum calcium: 14.1 mg/dL
Serum phosphorus: 2.1 mg/dL
Intact parathyroid hormone (PTH): 8 pg/mL (normal: 10-65 pg/mL)

A chest radiograph demonstrates a large, centrally located cavitary mass in the right lung. Which of the following best describes the underlying mechanism of this patient's electrolyte abnormality?

  1. A
    Ectopic conversion of 25-hydroxyvitamin D to its active form
  2. B
    Autonomous production of intact parathyroid hormone by the lung mass
  3. Tumor secretion of a peptide homologous to parathyroid hormoneAnswer
  4. D
    Extensive osteolytic destruction from skeletal metastases
  5. E
    Ectopic release of antidiuretic hormone leading to volume contraction

Answer

The correct answer is the tumor secretion of a peptide homologous to parathyroid hormone, known as parathyroid hormone-related peptide (PTHrP).
This patient presents with symptomatic hypercalcemia (confusion, weakness, constipation) accompanied by hypophosphatemia and a suppressed intact parathyroid hormone (PTH) level. In the setting of a central cavitary lung mass and a heavy smoking history, the most likely diagnosis is squamous cell carcinoma of the lung. Squamous cell carcinomas classically cause humoral hypercalcemia of malignancy by secreting parathyroid hormone-related peptide (PTHrP). PTHrP shares homology with the amino-terminal end of true PTH and binds to the same receptors in the bone and kidneys. This leads to increased osteoclastic bone resorption and increased renal calcium reabsorption, resulting in hypercalcemia. It also decreases renal phosphate reabsorption, causing hypophosphatemia. The severe hypercalcemia appropriately suppresses the normal parathyroid glands, leading to a low intact PTH level on standard laboratory assays (which do not detect PTHrP).

Step-by-Step Solution

1
Analyze the laboratory results in the context of the clinical symptoms.
The patient has severe hypercalcemia, hypophosphatemia, and an appropriately suppressed intact PTH level.
This establishes that the hypercalcemia is independent of the parathyroid gland (PTH-independent hypercalcemia).
2
Correlate the metabolic abnormality with the imaging findings and patient history.
The patient has a significant smoking history and a central cavitary lung mass, which is highly suspicious for squamous cell carcinoma of the lung.
Different histological subtypes of lung cancer are associated with specific paraneoplastic syndromes.
3
Identify the primary mechanism of hypercalcemia associated with this specific tumor type.
Squamous cell carcinoma frequently secretes PTHrP.
PTHrP binds to PTH receptors in the bone and kidney, mimicking the action of true PTH (causing hypercalcemia and hypophosphatemia) while suppressing endogenous intact PTH production.

Key Concept

Humoral hypercalcemia of malignancy is most commonly caused by the ectopic secretion of parathyroid hormone-related peptide (PTHrP), classically seen in squamous cell carcinomas (e.g., of the lung, head, and neck).
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Neoplastic Processes

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